The present invention relates generally to manufacturing process monitoring systems and, more particularly, to a system and method for predicting stud weld quality by monitoring various parameters including linear displacement of the stud carrying shaft during the welding process.
A typical arc stud welding system includes a welding gun and an associated power supply and controller for the gun. The gun generally includes a gun body having a shaft extending outwardly therefrom which is mounted for reciprocal movement with respect to the body, between fully retracted and fully extended positions. The gun shaft is typically moved axially via the combination of an electric solenoid and a biasing spring, both held within the gun body. The movable shaft has a distal end having means such as a chuck for carrying the stud to be welded to the workpiece. The gun may also include one or more stationary legs, each of which may be fixedly connected to an annular ceramic ferrule which is mounted so as to substantially surround the stud, with the stud projecting axially slightly beyond the ferrule.
To begin the weld process the stud is brought into contact with the workpiece, with the shaft in its fully extended position. The stud carrying shaft is then compressed axially against the biasing spring until a bottom surface of the ceramic ferrule is also seated on the workpiece. An electrical current is applied to the shaft and stud in order to establish an arc between the stud and the workpiece. Thereafter, the stud is then "lifted" a predefined distance away from the workpiece, via actuation of the solenoid, into a fully retracted position of the shaft. The electrical arc creates a molten pool of metal on the workpiece surface into which the stud is to be attached and, at the same time, also begins to melt the metallic stud.
After a predetermined arcing time has elapsed, the controller deactuates the solenoid and the shaft is forcibly driven by the biasing spring so as to "plunge" the stud into the molten pool and thereby effect welding. The plunge actually forces the stud into the molten metal below the original surface of the workpiece and the length of the plunge is essentially equal to the amount of "lift" plus the distance the stud is allowed to travel below the original surface of the workpiece. The depth of penetration of the stud below the original workpiece surface typically varies with current magnitude, arc time and certain other plunge parameters.
The manner in which the weld between the stud and workpiece forms ultimately determines weld quality, and in particular the impact strength of the weld, i.e. whether the joint between the workpiece and stud is sufficient to adequately secure the stud to the workpiece in a stressed condition, such as upon the application of a striking force directly to the stud. The force to be withstood may vary with application but typically must approach a maximum in order for the weld to be considered to be of a "good" quality. This impact strength can be physically tested using a destructive testing technique such as repeated hammer blows and the results of such tests have traditionally been correlated to various weld parameters including weld current, arc time and voltage. During a given welding process, these parameters may be monitored and compared to certain predetermined ranges. Generally, when the measured parameters fall within these standard nominal ranges for each of the parameters, a good quality weld is predicted. When monitored conditions fall outside of the set ranges, the resultant welds are predicted to be of a poorer quality and adjustments to the welding system are usually made until predicted and tested "good" quality welds are produced.
However, in practice it has been found that while monitored conditions falling within certain parameter ranges generally results in a good quality weld, at times a poor quality weld has resulted even when all of the conventional parameters have been measured and have fallen well within the set acceptable ranges. Welds made in accordance with these prescribed parameters have from time to time failed physical impact tests, sometimes by a relatively large margin, thereby allowing poor quality welds to pass through these conventional process monitoring methods. While an operator of a manual welding station may be trained to visually spot certain indications of poor weld quality, such as an irregularly shaped or insufficient flash, this type of manual inspection is not infallible nor is it cost effective in an automated welding operation.
Probable causes for insufficient weld quality occurring in instances wherein all of the traditionally monitored parameters have fallen within a generally acceptable range may include mechanical malfunctions of the gun such as improper set-up, adjustment or alignment. Similarly, improper lift and plunge distances can likewise cause an unreliable weld even when all measured parameters fall within prescribed ranges. Therefore, there exists the need for an improved system and method for more accurately predicting stud weld quality without necessitating any visual or other inspection of the completed weld.